Chlorinated elastomers may be cured by numerous means including the use of peroxide/coagent systems, thiadiazole-based systems, or irradiation crosslinking techniques. Peroxide cures are typically preferred for their scorch safety, shelf-life or bin stability, low permanent set, and high temperature performance. However peroxide cure systems are often unacceptable for use in molded goods because of mold sticking and fouling due to volatiles or in applications that require a low temperature cure due to equipment or processing limitations.
Thiadiazole-based cure systems provide certain advantages such as the ability to cure over a wider range of temperature and pressure conditions than peroxide cures while generating fewer volatile byproducts, having good mold release characteristics, and the ability to use less expensive compounding ingredients such as aromatic oils.
Despite the excellent vulcanizate properties obtainable with the polymercapto/inorganic base/accelerator-based vulcanization systems for chlorinated elastomers, two key problems remain that limit their usefulness. The first problem is the premature vulcanization of the compound during storage (i.e. bin stability) or processing (i.e. scorch safety) prior to forming the vulcanized article, and the second problem is the inconsistency of the vulcanization rate. It is desired that the elastomer compound, after preparation, be stable under typical environmental conditions of storage and during processing of the compound such that minimal vulcanization occurs prior to formation of the finished article. A consistent vulcanization rate is desired, so that processes that form the finished vulcanized article, e.g. injection molding, extrusion, or compression molding, can be run consistently, and without the generation of scrap materials.
U.S. Pat. No. 4,745,147 discloses curable compositions of a chlorinated polyethylene, a polymercapto crosslinking agent, an acid acceptor, and a polyhydric alcohol. The composition must also contain a cure initiator such as an amine, a quaternary ammonium salt or a quaternary phosphonium salt. The onium salts are of either the formula R1R2R3R4Z+ Cl− or the chloride salt of an N-substituted pyridinium ion, wherein Z is a nitrogen or phosphorous atom; R1-R4 may be the same or different from each other, and like the N-substitution on the pyridinium ion, are selected from C1-17 alkyl, cyclohexyl, phenyl and benzyl groups. The polyhydric alcohol is said to improve the cure state and heat aging of crosslinked compositions.
Despite the research efforts in developing and improving in the bin stability, scorch safety, and cure rate reproducibility, there is still a need for further improvement in bin stability, and scorch safety while maintaining an acceptable cure rate.